The most important components of photovoltaic cells are the semiconductor layers. The semiconductor layers are where sunlight is converted into electrical energy. More particularly, semiconductors harness the energy from sunlight and convert it into electrical energy. Sunlight is composed of particles of energy known as photons. These photons contain different amounts of energy depending upon the wavelength of light. The entire wavelength spectrum of sunlight from infrared to ultraviolet light has an energy range of 0.5 eV to 2.9 eV, respectively. When these photons hit the surface of the solar cell, the photons may be absorbed, reflected, or pass through the solar cell. The energy from those photons that are absorbed by the semiconductor atoms is transferred to the atoms' electrons. The added energy from the photons excite the electrons within the solar cells thereby enabling the electrons to break away from their associated atoms and become part of the electrical current. The amount of energy required to “liberate” the electrons from their atomic bonds to produce electrical current is known as the band-gap energy. Different semiconductor materials have differing band-gap energies. In order to liberate these electrons from their atomic bonds, a photon must have an energy equal to the band-gap energy. If the photon of light lacks sufficient energy, the photon passes through the material or generates heat. Given the particular energies required to generate an electrical current, approximately 55% of the sunlight's energy cannot be utilized. Furthermore, there is no one ideal material that may be used to capture energy from the sunlight's broad energy spectrum. Thus, there has been a desire to find semiconductor materials with broad band-gap energies.
Traditionally, the semiconductor layer of photovoltaic solar cells has been made from crystalline silicon. Crystalline silicon is used in many forms such as monocrystalline, multi-crystalline, ribbon and sheet, and thin layer silicon. Typically, crystalline silicon solar cells are made of silicon wafers having a thickness ranging from 150-350 microns. Various methods are known for the production of crystalline silicon such as the Czochralski method, float zone method, casting, die or wiring pulling. Silicon-based solar cells are expensive because a large amount of raw material is required and the necessity to remove impurities and defects from the silicon. Techniques such as passivation (reacting surface with hydrogen) and gettering (chemical heat treatment that causes impurities to diffuse out of silicon) have been developed to address the problem of impurities and defects. Moreover, silicon has a band gap energy of 1.1 eV which is at the lower range of effective semiconductors. Furthermore, advancements in the use of crystalline silicon in solar cells has resulted in an increasing demand for solar grade silicon, yet the availability of such silicon is dwindling.
As a result, other semiconductor materials and technologies that may be utilized for fabricating solar cells have been sought out. In particular, thin film photovoltaic cells has garnered considerable attention and study in recent years. These cells are made of semiconductor materials that are only a few micrometers in thickness. Typically, these cells are comprised of two semiconductor layers. The two layers have different characteristics in order to create an electrical field and a resultant electrical current. The first thin film layer is commonly referred to as the “window” layer or negative type (n-type) semiconductor. The window layer absorbs high energy light energy, but it must also be thin as to let light pass through the n-type layer to the second semiconductor layer, the absorbing layer. The absorbing layer or positive type (p-type) layer must have a suitable band gap to absorb photons and generate high current and good voltage. Thus, less semiconductor material is required thereby reducing the costs of producing solar cells as compared to crystalline solar cells. Thin film photovoltaic cells have been developed using semiconductor materials such as amorphous silicon, cadmium telluride, and copper-indium-diselenide (CIS), and copper-indium-gallium-diselenide (CIGS).
In particular, CIGS has gained considerable interest and study in recent years. The focus of research with respect to CIGS has concentrated on developing low-cost manufacturing techniques for thin film CIGS. For instance, some of the earliest techniques involved selenization, which is the process of heating copper and indium on a substrate in the presence of a selenium gas. A drawback of this process is selenium-containing gas such as H2Se is highly toxic and presents a great health risk to humans in large scale production environments.
Other techniques used to form CIGS thin films includes sputtering techniques and physical vapor deposition (PVD). U.S. Pat. No. 5,045,409 issued to Eberspacher et al. discloses the deposition of copper and indium films by magnetron sputter and deposition of a selenium film by thermal evaporation in the presence of various gases. Methods such as PVD of single crystals and polycrystalline films yield highly efficient solar cells, but PVD is very expensive and difficult to scale up for large scale production. Thus, there is a need for producing inexpensive methods of producing highly efficient CIGS films for photovoltaic cells in large scale.